This invention in general is directed to a method for maintaining a substantially uniform and stable operation of a blast furnace and in particular to a blast furnace which is susceptible to erratic operation caused by the introduction of large disturbances into the furnace. The method is based on the differences in the average (HTH) and (CEEP) parameters and their values over a preselected period of operation. The differences are used to determine the changes to be made in the temperature and/or moisture content of the hot blast air introduced into the furnace.
High temperature heat, (HTH), is defined as the heat above about 1800.degree. F. (980.degree.C.) available in the tuyere region of the furnace to melt the burden, reduce the metalloids to their final state, reduce with carbon the FeO which has not been reduced by indirect reduction, heat the slag and hot metal to their final temperatures and be lost through the furnace wall. The tuyere region is defined as the lower portion of the blast furnace which includes the upper portion of the hearth wherein the tuyeres enter the furnace through the furnace wall, the tuyere raceway and the lower bosh.
The CEEP parameter is a linear function of the coke consumption ratio and reducing gas utilization factors. It is defined as: EQU CEEP={CCR-[(ETACO/90-1+[(ETAH2/90-1)/3])+0.6+1]}/2+1
The coke consumption ratio, CCR, is the ratio of the quantity of coke reportedly consumed in the furnace over a preselected time period divided by the quantity of coke actually consumed in the process. The reducing gas utilization factors, ETACO and ETAH2, are the amount of carbon monoxide and hydrogen gases, respectively, that are used to reduce the iron oxides to iron divided by the amount of each gas that is theoretically possible to reduce the iron oxides to iron.
The production of hot metal, for example basic iron and foundry iron, in a blast furnace is very complex and is dependent upon many variables, for example uniformity and quality of materials, such as iron-containing ores and pellets, carbon-containing materials such as coke, and fluxstone, such as limestone, dolomite and the like which constitute the burden charged into the furnace, the flame temperature, slag volume, slag basicity, wind rate, ore/coke ratio, etc. As the burden moves downwardly in the furnace, hot gases pass upwardly through the burden and reduction-oxidation reactions between the hot gases and the burden materials occur at various levels in the furnace. In the upper level of the furnace, the reducing agents are the gases carbon monoxide and hydrogen and the reduction reactions, which are referred to as indirect reduction, are slightly endothermic. Reduction by solid carbon occurs in the lower stack of the furnace and this reaction, referred to as direct reduction, is highly endothermic. The heat necessary in the lower part of the furnace to melt the iron and slag and complete the reduction reactions is a function of the ore/coke ratio and the completeness of the indirect reduction reactions which occurred in the upper level of the furnace.
The reduction of silica to silicon, which occurs in the lower level of the furnace, is highly endothermic. Because only a portion of the silica charged into the furnace is reduced to silicon in the blast furnace by the endothermic reaction, any increase in the difference between the available heat and the necessary heat results in higher silicon levels in the hot metal. Stated more simply, as the high temperature heat in the furnace increases, or the necessary heat in the lower part of the furnace, as measured by the parameter CEEP decreases, the silicon content in the hot metal increases. Conversely, as the high temperature heat in the furnace decreases or the heat necessary in the lower part of the furnace increases, the silicon decreases. Therefore, knowing CEEP the high temperature heat in the furnace can be controlled to produce hot metal having a uniform chemistry.
Mass and heat balance calculations applicable to the operation of a blast furnace have been developed over the years to predict furnace performance. The mass and heat balance calculations can be solved manually and could be used by operators as a guide in the manual control of the furnace. The data collected are voluminous and much time is required to manually obtain mathematical solutions of the balances. It was, then, only natural that with the advent of computers, furnace operators would begin to use the computers to make the mass and heat balance calculations and use the results to aid in the control of the furnace.
In the past twenty or so years many feedforward and feedback control schemes have been proposed to control and to maintain a uniform operation of the furnace. Feedforward control schemes are designed to prevent the occurrence of disturbances in the furnace. Feedback control schemes are designed to reduce the effects of minor disturbances which occur in the furnace.
Feedback control schemes that use top gas data have been employed with varying degrees of success. Such schemes when used with ironmaking furnaces having raw material bedding and blending facilities have been used successfully. One such control scheme is described in U.S. Pat. No. 4,227,921 issued Oct. 14, 1980 to Yoshiyuki Matoba et al. entitled "Method of Controlling a Blast Furnace Operation" and teaches a method for controlling a blast furnace operation based on the assumptions that the working volume of the furnace can be vertically subdivided into a plurality of horizontal zones wherein in each zone predetermined reactions proceed uniformly, the amount of material in each zone does not vary significantly and reaction rates such as .eta.H.sub.2 are stable.
Another method for controlling a blast furnace is described in an article entitled "Development and Application of Computer Control System at Sakai No. 2 Blast Furnace" by Yajiro Fukagama et al. in the Proceeding ICSTIS Section 1 Suppl. Trans. ISIJ, Vol. 11, 1971, 143-147. The article teaches that a uniform and stable operation of a blast furnace can be achieved by the use of a computer control system based on a theoretical coke temperature determined from heat balance calculations which use accurate top gas analyses. However, as stated in the article, primary emphasis is placed on burden preparation because of its fundamental importance in stabilizing furnace conditions. It is necessary to maintain uniformity of the burden charged with regards to composition and weight. Consequently, such schemes when used with basic iron furnaces which are not equipped with bedding and blending facilities have not been successful and have been abandoned.
The results of a 21/2 year study of blast furnace control in the blast furnace of August-Thyssen Steel at Ruhrort are detailed in the final report of a European Communities Commission, European Coal and Steel Community, Steel Research Reports, Pig Iron Production and Direct Reduction, "Automation of the Blast Furnace Process, Part 1: Plant Trials with Blast Furnace Control at the August-Thyssen Heutte A.G." issued in 1976. The work was done under Research Contract No. 6210-30/1/071 and was published under the auspices of the general directorate of Scientific and Technical Information and Information Management. The report shows that feedback control systems based on mass and heat balance calculations which use top gas analysis are not successful if large disturbances caused by inconsistent burden properties are introduced into the furnace. The result is not surprising in view of Staib et al., pioneers in the use of mass and heat balances for control of a blast furnace, who in their paper "Theoretical Considerations on the Automation of Blast Furnace Operation" presented at the Conference on Automation in Steelmaking, Amsterdam and Dusseldorf, Mar. 29-31 and Apr. 1-3, 1965 (English translation--BISI #4680, March 1966) stated that it is essential that the blast furnace burden composition be kept relatively constant because errors made in charging the furnace cannot be compensated for by heat control that is based on a calculated term. They further state that their techniques are valid only when the mass and heat transfers have a substantially steady-state nature.
In an article entitled "Burden Preparation and Computer Control of the Blast Furnace," in Journal of Metals, March 1968, pp. 68-74, J. M. VanLangen et al. also state that in using a calculated high temperature heat term for control it must be assumed that the furnace operates in a stationary state, i.e., burden composition and temperature are only a function of place and not of time. They further state that this implies that the ore burden composition and the coke to ore ratio are constant.
Also, Japanese reports by Fukagama et al. and Matoba et al., hereinbefore mentioned, base their feedback control methods on the calculated solid temperature in the lower part of the furnace and also assume that the furnace operation is uniform, i.e., free from the introduction of large disturbances into the furnace.
It has been concluded that feedback control schemes based on mass and heat balances can only be used when the necessary heat and the reducing gases required per ton of iron oxides reduced are constant. These requirements can only be met with a burden charge of raw materials having uniform compositions which add little if any "noise" (changes in variables, such as the physical and chemical properties of the burden charged through the top of the furnace) to the furnace operation.
There is, therefore, a need for a feedback control scheme that is capable of maintaining a substantially uniform operation of a blast furnace susceptible to the introduction of large disturbances.
It is an object of this invention to provide a feedback control scheme for maintaining a substantially uniform operation of a blast furnace susceptible to the introduction of large disturbances wherein accurate top gas data are continuously obtained and stored in a computer and averages of the top gas data are determined periodically and are used to compute the values of high temperature heat (HTH) and CEEP in the furnace by means of mass and heat balance calculations. The periodically determined values of high temperature heat (HTH) and CEEP are stored in the computer and averages of these periodically determined values are determined for preselected periods of blast furnace operation. The periodically determined values for high temperature heat (HTH) and CEEP are compared to their respective averages and these differences are used to determine changes which may be required to be made in the temperature and/or moisture content of the hot blast air to thereby maintain the aforementioned substantially uniform operation of the blast furnace and to produce a high quality hot metal having a consistently uniform chemistry characterized by a silicon content within a preselected range. The preselected period of operation for the average high temperature heat calculation is the most recent period during which the hot metal produced is characterized by a silicon content which is within a predetermined range of the aim silicon content. The preselected period of operation for CEEP is the period prior to the current period which could be as much as 24 hours or as recent as 9 hours.